This invention relates to a method of recovering alkali values from trona ore. In particular, this invention relates to a process for producing sodium carbonate via sodium bicarbonate crystallization.
Trona ore is a mineral that contains about 90-95% sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O). A vast deposit of mineral trona is in southwestern Wyoming near Green River. By conservative estimates, this deposit contains about 75 billion metric tons of trona ore.
The sodium sesquicarbonate found in trona ore dissolves in water to yield approximately 5 parts by weight sodium carbonate (Na.sub.2 CO.sub.3) and 4 parts sodium bicarbonate (NaHCO.sub.3). To recover these valuable alkali products, the trona ore must be processed to remove insoluble materials and other impurities.
One such valuable alkali produced from trona is soda ash (a commercial grade of sodium carbonate). Soda ash is one of the largest volume alkali commodities made in the United States. Soda ash finds major use in the glass-making industry and for the production of baking soda, detergents and paper products.
A common method to produce soda ash from trona is known as the "monohydrate process", which requires large quantities of water (a scarce and valuable resource in Wyoming) and energy. In that process, crushed trona ore is calcined (i.e., heated) at a temperature between 125.degree. C. and 250.degree. C. to convert sodium bicarbonate into sodium carbonate, drive off water of crystallization and form crude soda ash. The crude soda ash is then dissolved in water and the insoluble material is separated from the resulting solution.
This clear solution of sodium carbonate is fed to an evaporative crystallizer where some of the water is evaporated and some of the sodium carbonate forms into sodium carbonate monohydrate crystals (Na.sub.2 CO.sub.3.H.sub.2 O). The monohydrate crystals are removed from the mother liquor and then dried to convert it to dense soda ash. The mother liquor is recycled back to the evaporator circuit for further processing into sodium carbonate monohydrate crystals.
The monohydrate process has several disadvantages. Presently, the monohydrate process consumes considerable amounts of water. When the raw trona ore is calcined, the natural water content is evaporated. Yet after that water is evaporated off, more water must be added to dissolve the calcined trona.
In addition to consuming water, the monohydrate process consumes and wastes considerable amounts of energy. Calcining equipment, for example, has only about 50% energy efficiency. Much energy is also wasted in calcining simply to raise the temperature of the sodium carbonate in the trona up to the calcining temperature even though the sodium carbonate itself does not undergo conversion. Additionally, a significant amount of energy is wasted during calcining in evaporating the naturally occurring water in the trona.
Soda ash may also be produced from the "bicarbonate process." One advantage over the "monohydrate process" is that in the "bicarbonate process" the raw trona ore is not calcined. Rather, trona ore is first dissolved and then fed to a carbon dioxide absorber tower to convert the sodium carbonate dissolved from the trona into sodium bicarbonate. Then some of the sodium bicarbonate is crystallized and removed from the resulting mother liquor. The sodium bicarbonate crystals are calcined to convert to soda ash. The mother liquor is recycled to dissolve more trona.
Unfortunately, the bicarbonate process wastes energy by requiring a high liquor recycle ratio to dissolve the trona. This inefficiency is due to the high levels of sodium bicarbonate remaining in the mother liquor after the crystallization step. This is a limiting factor in the amount of trona that can be dissolved in it.
Therefore, there is a need for a process that avoids or minimizes the waste and inefficiencies of the traditional processes for making soda ash.